The present invention relates to the synthesis of the metastable crystal phase of vanadium oxide known as VO.sub.2 (B), and to the manufacture of electrochemical cells having cathodes incorporating this material.
In recent years, there has been substantial progress in development of non-aqueous rechargeable or "secondary" electrochemical cells for use as electrical storage batteries. Considerable effort has been directed towards development of cells useful in small electronic applications such as calculators, circuit boards and watches, as replacements for nickel cadmium cells. Non-aqueous secondary cells typically include a metal anode, an electrolyte incorporating a salt of the metal, and a cathode incorporating an active material capable of reversibly taking up the metal. When the cell is discharged, metal leaves the anode, passes through the electrolyte and is taken up by the cathode with release of electrical energy. When the cell is recharged, the metal is released from the cathode to the electrolyte and redeposited back onto the anode. The cell thus stores electrical energy as chemical energy during recharge, and releases that stored energy as electrical energy during discharge.
One aspect of secondary cell research has been directed towards cells incorporating cathode-active materials defining host crystal lattice structures that undergo reversible oxidation-reduction reactions with intercalated metal ions. In particular, cells incorporating anodes of alkali metals together with cathode-active materials having crystal lattices formed from transition metal oxides demonstrate higher energy densities than many other cells. Accordingly, considerable effort has been directed towards the search for particular transition metal oxides useful as cathode-active materials in secondary cells.
The transition metal oxides which have been studied, include certain vanadium oxides. The vanadium-oxygen phase diagram is depicted in G.E. Moffat, The Handbook of Binarv Phase Diagrams, Genium Publishing Corp., Schenectady, N.Y. (1986), the disclosure of which in said handbook is hereby incorporated by reference herein. VO.sub.2 is bordered on the oxygen-rich side by compounds of the form V.sub.n O.sub.2n+1 (n&gt;2) (e.g., V.sub.2 O.sub.5, V.sub.3 O.sub.7, V.sub.6 O.sub.13 . . . ) having tetragonal symmetry in the rutile phase, and on the oxygen poor side by compounds of the form V.sub.n O.sub.2n-1 (n&gt;3) (e.g., V.sub.3 O.sub.5, V.sub.4 O.sub.9 . . . ) having monoclinic symmetry. VO.sub.2 is the point at which the transition from the oxygen rich series to the oxygen poor series. Accordingly, VO.sub.2 occurs in a rutile (VO.sub.2 (R)), monoclinic (VO.sub.2 (M)) and a transitional, metastable state (VO.sub.2 (B))
At all temperatures, VO.sub.2 (B) has a higher free energy than VO.sub.2 (R) or VO.sub.2 (M). When VO.sub.2 (B) is heated above about 537.degree. C., it undergoes an irreversible, exothermic transformation to VO.sub.2 (R). This high temperature is needed to overcome the activation energy between the phases. VO.sub.2 (M) forms when VO.sub.2 (R) is cooled below 67.degree. C. The transition between the R and M phases are reversible. For T&gt;67.degree. C., the R phase is stable and for T&lt;67.degree. C., the M phase is stable.
Because VO.sub.2 (B) transforms to VO.sub.2 (R) at about 537.degree. C., it cannot be synthesized directly from vanadium and oxygen. It must be produced by the alteration of a closely related material at a temperature low enough to prevent the transition to VO.sub.2 (R) from being activated. At room temperature the VO.sub.2 (B) structure, once formed, is retained indefinitely. Only upon heating to about 537.degree. C. does it transform to the rutile phase.
Christian et al., U.S. Pat. No. 4,228,226 describes secondary cells utilizing as cathode-active materials certain vanadium oxides having the nominal stoichiometry VO.sub.2 +y, with y between 0.0 and 0.4. The vanadium oxides within the stoichiometric range taught by Christian et al. include VO.sub.2, V.sub.6 O.sub.13, V.sub.4 O.sub.9 and V307 The structures of these materials are discussed in detail in Acta Chemica Scandinavica 25, pp. 2675-2687 (1971) and Acta Crystallographica, A33, pp. 834-837 (1977).
Christian et al. offers a general teaching of the synthesis of the vanadium oxides having the nominal stoichiometry VO.sub.2+y, with y between 0.0 and 0.4, by reaction of V.sub.2 O.sub.5 with appropriate quantities of vanadium metal or V.sub.2 O.sub.3 at high temperatures in vacuo, by reduction of V.sub.2 O.sub.5 or NH.sub.4 VO.sub.3 with gaseous reducing agents such as H.sub.2, NH.sub.3 or SO.sub.2, and by thermal decomposition of NH.sub.4 VO.sub.3 in an inert atmosphere. With regard to the VO.sub.2 (B), the reference offers a specific teaching of reduction of V.sub.2 O.sub.5 with H.sub.2 at 325.degree. C. The H.sub.2 reduction of V.sub.2 O.sub.5, however, is extremely temperature sensitive and requires several days for completion. At 350 C V.sub.2 O.sub.3 is formed when the reduction proceeds for three days and at 320.degree. C., almost no reduction occurs at all after three days.
Abraham et al., J. Electrochem Soc. 128, 2493 (1981) discloses methods similar to those of Christian et al. to produce VO.sub.x compounds with x between 1.88 and 2.22, in which NH.sub.4 VO.sub.3 is decomposed under an inert gas flow near 450.degree. C.
The thermal decomposition products of NH.sub.4 VO.sub.3 are discussed in detail in Thermochimica Acta 36, pp. 287-297 (1979). Soviet Pat. Application No. 327,795 describes the preparation of V.sub.6 O.sub.13 by the reduction of V.sub.2 O.sub.5 by SO.sub.2. The isothermal reduction of V.sub.2 O.sub.5 by SO.sub.2 gas to the phases V.sub.4 O.sub.9, V.sub.6 O.sub.13 and V.sub.2 O.sub.4 was also described by KaWashima et al.. Chemistry Letters, pp. 1131-1136 (Chem. Soc. of Japan, 1975). Hammou et al., U.S. Pat. No. 4,619,822 describes the preparation of V.sub.6 O.sub.13 by the reduction of V.sub.2 O.sub.5 in a CO/CO.sub.2 atmosphere. Riley, U.S. Pat. No. 4,486,400 describes the preparation of stoichiometric V.sub.6 O.sub.13 by the decomposition of NH.sub.4 VO.sub.3 to obtain non-stoichiometric V.sub.6 O.sub.13, which is then heated in a dynamic CO/CO.sub.2 or H/H.sub.2 O atmosphere having an oxygen partial pressure equal to the oxygen partial pressure over stoichiometric V.sub.6 O.sub.13 at the heating temperature, to yield stoichiometric V.sub.6 O.sub.13 Riley does not discuss the synthesis of VO.sub.2 (B).
Theobald et al., J. Solid State Chem. 17, pp. 431-438 (1976), describe the preparation of VO.sub.2 (B) by reaction of V.sub.2 O.sub.5 with a reducing gas preferably H.sub.2, although NH.sub.4 is also disclosed. Theobald et al. also states that VO.sub.2 (B) can be prepared by decomposing (NH4).sub.2 V.sub.6 O.sub.16 and other various vanadates of ammonia, in sealed tubes, but that this method is of poor reproducibility. Murphy et al., J. Electrochem Soc. 128, pp. 2053-2060 (1981), describes the preparation of VO.sub.2 (B) by reducing V.sub.2 O.sub.5 under a hydrogen flow at 320.degree. C. followed by drying in Ar to remove residual water.
Cells assembled with VO.sub.2 (B) cathodes and lithium anodes operate at a voltage close to 2.45 volts throughout the discharge of the cell. Because this is about twice the voltage of a nickel cadmium cell, such Li/VO.sub.2 (B) cells could be used as direct replacements for nickel cadmium cells. Thus, a single Li/VO.sub.2 (B) cell can replace two nickel cadmium cells in series. Although Li/VO.sub.2 (B) cells have desirable voltage characteristics, the Christian et al. patent discloses that non-aqueous lithium secondary cells having cathodes comprising VO.sub.2 (B) suffer a 35% loss of energy capacity after only 15 charge/discharge cycles. By contrast, Christian et al. indicates that cells incorporating other cathode-active vanadium oxides do not suffer such rapid loss of energy capacity Accordingly, VO.sub.2 (B) has not been widely adopted as a cathode-active material in secondary cells.